Recently, microwave multistage amplifiers used in a microwave communication systems have been required to have significantly high stability in providing operation essentially free from abnormal oscillations, which may be caused by environmental conditions or conditions of any other device connected to the amplifier. A solid-state element such as HEMT, MES-FET or the like used in the band range of the recent microwave has a high power gain, and when a microwave multistage amplifier is constructed by using such solid-state elements and there is no sufficient spatial isolation between the input terminal of the multistage amplifier and the output terminal of the same, there may be a fear of causing an abnormal oscillations of the multistage amplifier.
Referring to the attached drawings, a conventional microwave multistage amplifier of the above-mentioned type will be described below.
FIG. 5 shows a microwave multistage amplifier according to a prior art, and FIG. 6 is a sectional view thereof. In FIGS. 5 and 6, numerals 1, 2 and 3 denote micro strip lines, and numeral 4 denotes a dielectric substrate for forming a microwave circuit thereon. A microwave signal inputted through an input terminal 9 is sequentially amplified by microwave amplifiers 5 and 6, and then is outputted through an output terminal 10. The microwave signal amplified by the microwave amplifier 6 has a potential level higher than that of the microwave signal inputted through the input terminal 9. Therefore, under certain conditions of permittivity or thickness of the dielectric substrate, the microwave signal outputted from the microwave amplifier 6 is possibly transmitted to the micro strip line 1 as a surface wave mode or a strong spatial radiation, thereby causing an abnormal oscillation of the multistage amplifier.
In order to prevent the above-mentioned adverse phenomenon, it is required to provide spatial isolation between the input side and the output side of the microwave multistage amplifier. In detail, there are provided a partition plate 7 which is screwed or soldered to a earth pattern 16 mounted on the dielectric substrate 4 by means of a through hole 15, and an upper cover plate 11 which is fixed to the shield casing 8 by means of screws 12 and 13 for closing the opening of the casing 8.
In the above-mentioned structure, however, the upper cover plate 11 is not permitted to be screwed to the partition plate 7, because this screwing causes a deformation of the dielectric substrate 4 and, as a result, a crack in the earth pattern 16. In consequence, a gap remains between the upper cover plate 11 and the partition plate 7, which deteriorates the spatial isolation between the partitioned rooms in the casing.
Further, there exists a problem that a microwave is transmitted through in the dielectric substrate 4 in one room of the casing as a surface wave mode, excluding a TEM mode, and is transferred to the micro strip line 1 in the other room of the casing, thereby also deteriorating the isolation.
The object of the present invention is to overcome the above-mentioned problems, and to provide a microwave multistage amplifier having an excellent operational stability, in which any isolation deterioration due to the generation of a surface wave mode wave transmission through the dielectric substrate is prevented by strictly separating the rooms in the casing from each other.